Manufacture of crystalline anhydrous alkali metal orthosilicate



June 8, 1948. B. K. BEECHER MANUFACTURE OF CRYSTALLINE ANHYDROUS ALKALI METAL ORTHO SILIGATE Filed July 24, 1944 INVEN roe BBflZ/EE K. BEECHEE By M W 7 ATTOE/VE T Patented June 8, 1948 'MANUEACTURE LIOF CRYSTALLINE 'ANHY- P-DRQUSwALKALLMETAL ORTHOSILICATE IBrazier K. Beecher, Wyandotte, Mich., assig nor 'towyandotte Ghemicals Corporation,- a corpo- Application July. '24, 1944, Serial No. 546,341

. i1 JThGL'PF-GSQIlt11137811 01011: relates-to the manufacture of anhydrous crystallinehalkal-i.metal sililcatesyli, .e.,- s.odium,.pot.assium ,or"1ithium orthoisilicates, from molten caustic and. either silica or ri the' soluble alkali imam silicates. The ratio of e.

,.-:MezO;SiOz .(Me. representing, either" Naj'K or'lLi) in these soluble silic'ateausedas a source of silica lisless than that in. sodium ,ortho'silicate, i. e., less lgthani2f;1. Suchjsilicates which are reacted em 3Glaims. (01. 23-110) o ,Fig. .3. is a view similar to Fig.2 but at 65 diameters magnification.

In Fig. 1 the numeral? represents the X-ray diffraction pattern of orystals'of anhydroussodiumt orthosilicate made by applicants --process. .The numeral l; represents theX=raytdi'ffraction ..pattern,of a commercial product; referred" to in scommer'cial channels ,as sodium orthosilicateand .brace the. anhydrous silicates as [glasses or pow-" Q dersj hydrous solidsandthe solutions. These a1- kali metal silicates -and their preparations are described on pages"108'-127 oi. 'Vails Soluble'Silicates in Iridustry',#18 monograph series1928 -edition. $pecific'; examples'are sodiummetasilh fcate'Na'z'SiOa and sodiumdi'silicate Nazsi'uos. "My invention relatesimoreover; to-themanufacture of crystalline anhydrous orthosilicates 'of" a white J'sha'de or substantiallywhiteg meaninghavingbut a. slightly igreenxgraytor' creamtinge. Blue 1 anhydrous oithosilicateigtdwhich I" refer his the crystalline ,orthosilic'ate' firstdes-cribed in the article Jordis iriZjAnorg'fChemb 58 984.01

,This' crystalline, orthosili'catef is not to be com- I 'fused with the, so-called sodium: ortho silicate" or si1bsilicate's which have'beengproducewby the Z tollowing' threedi'fferent methods:

i (1)Thewetdigesition' method whereinsilica or water glass .is;d ,iges.ted withaqueousica'ustic alkali solution, illustrative of which is the British Patent #3911407 01:1933;

"'(2)';T-l1efus-i0n of furnacemrooess where an al- ,kali-' salt,.such as. sodium' carbonate, chloride or sulf ate, :is heated--with silica 1170" a temperature above the "melting point of, the reaction product;

"and

(3) The dry'process-wherein caustic-alkali and silica are: mixed together; subjected to a tempera- .turebelow the meltingpoint ofthe caustic, illus- 40 trative 0f which are the' -Beecher- U. "'S. Patent $2,219,646 and the Soderburg U. S. Patent $2,211,733.

v "The commercial products -from these respective 'above outlined processes, Where to the solid form which was made by a process referred to supra (3) .type. By comparison, of-these-respective patterns ,it .Will .he seen that .the" lines characterizing 2, do not appear in 1;. the-latter exhibitsthe .lines which. appear in=1:hotographs of the" X ray [diffraction pattern of a "mixture of sodium-hydroxide and anhydrous sodiunr meta-silicate.

Acco-rdingly' the i conditions f orthe formation of sodium orthos'ilicate ashdisclosed. in the-J ordis -a1tticle referredto herein "wouldbe" entirely imvprvsrhtic'al y from has commercial l standpoint. It" is clear fromqa study. of the Jordis procedure that the fusion of silica with caustic-"soda was conducted With laboratory grade, pure" caustic and in "a silver crucible withoutstirring. No agitation or agitation only. for the-duration of the dispersion of the ground-silica'into' themolteneaustic tends to yield .a slurry'whiclr'is sonon-ur'iiform l and furnishes so few crystals of a'largesize that the slurry cannot be satisfactorily handled.

Commercial manufacture in quantity of sodium orthosilicatelis obviously-limited to the use of 1 technical grade causticsodaand to the use 'of .Vessels of gray or'cast iron, such'as-employed' in theldehydration of; aqueous caustic soda. While "theoretically 'vessels constructed of the noble metals might beused' to avoid" c'ontaminationsof caustic.

My present invention relates to a process; operable upon a commercialtonnage scale, wherein the alkali metal ortho'silic'a-tes of anhydrous crystalline character are formed by rea'ctingcommercial caustic soda or p' otash in a -m'olten condition with either silica or the soluble alkali metalsilic ates and driving offthe water of reactiontthereby precipitating the orthosilicates in a white-or substantially White condition. To secure such superior grade crystals-it is-necessary thatcaustic soda in the-s0 termed'green= state (i.- e., of: a greenish color)'=be*used. 'C'austioi-n this condition is Well-known tothe worker on the' 'fin-iShingpQtS :in the manufacture of: caustic soda', who; however,

adds sulphur '5 before cooling Y to eliminate this colon-page 29550:? iiManufa'cture ofesodajxby T. P. Hou (A. C. S. monograph series #65, 2d edition). The molten green caustic may be obtained either by transferring from a pot of the molten, material or by melting the green solidified caustic.

It is essential that the molten caustic be in the green state before and during the formation of sodium orthosilicate, in order that the iron picked up by the caustic from the walls of the vessel be kept in soluble form. On the other hand whencaustic is in the white or shaded state the iron picked up from the iron vesselduring the course of the reaction occurs in the form of a reddish brown precipitate which is occluded by the orthosilicate crystals during precipitation with the result that the crystals are of a brown color and contain iron in excess of 100 parts FezOa per million. Upon prolonged standing in the pot the initially green molten caustic will ultimately be-- come reddish brown in color and turbid. However, precipitation of sodium orthosilicate from an entire run of a full charge of the caustic can be completed and removed from the vessel long before the iron precipitates.

While it is preferred to add the silica'or, the alkali metal silicate in the anhydrous condition to anhydrous molten caustic, thesilica or the silicate can be added with water or the caustic may contain water or both, provided not only the water of reaction, but also the accompanying water is substantially removed. There is a very fine precipitate formed in the initial stages,

and as the dehydration proceeds granular alkali metal orthosilicate becomes distinct in the molten caustic which becomes clearer and freer of;

water. It will be understood that the caustic is in excess over the theoretical amount required to combine. with the, silica or silicate according to the reaction of this process, which is represented in the case of silica by the following equation;

4NaOH+SiOz:- Na4Si0-r+2HzO. Water is expelled by the temperature at which the molten caustic is maintained and this occurs not only when'the silica encounters the molten caustic,

but during the gradual expulsion of water of re-,;;

action. When the find precipitate has become granular and rapidly settles out, there is no or substantially no water present in the molten caustic I alkali. Y I

g In order to secure crystals of sodium orthosili-ci- 1 cats which are uniformly large, it is important that the pulverized silica should be added. at

spaced intervals. The intervals between additions should be spaced so that time is allowed both for the evolution of water vapor and the in a short space of time, agitation is continued until these small particles have grown sufficiently in size to become granular. At this time another addition of pulverized silica is added and agitation continued until the same phenomena are? observed. A convenient test is to remove a small ladle (about pint capacity) mixture from the pot and note the time required for the aforesaid settling. I have found-that a serviceable guide to follow is that the crystals should settle in this; ladle in approximately seconds by visual inspection. This addition of silica or silicates, ex polling of water of. reaction and stirring to condition the crystals is repeated, until the desired amount-of silica has been added. I

No precise time limit for the intervals between the addition of silica can be given as this depends upon many factors. The higher the temperature, the more rapid is the disappearance of the opaque, finely divided precipitate and the formation of the-discrete crystals. The greater the degree of agitation the shorter isthe interval between "additions. The smallrthe fraction of the total silica added, the shorter is the time between additions. In all events the important criterion is the substantial disappearance of the very fine, readily suspended particles initially formed upon the addition of the silica before the addition of succeeding batches of silica. The effect of my method of operation, therefore, is to 'pass repeatedly through the crystallization step, while limiting the number of nuclei for the crystal growth during each successive addition, in order that the desired size of crystals may be obtained. There is, of course, water present when the small solid particles are first-formed, but this is substantially removed by the time the discrete crystals appear. 7

The size of the individual additions may be varied over wide limits. I prefer to make the additions in units of about 15% of the total silica to be added. These units can be much larger, such as 25%, in which case more time must be allowed between additions and smaller crystals can be expected. On the other hand, the individual additions can .be much smaller with less time between additions.

The total amount ofsilica to be added to a given amount of caustic maybe varied over a wide range, depending on the physical characteristics of the finished slurry that is desired. By

reference to the equation representing the reaccaustic as disclosed in the example below, 263

pounds of molten caustic will be left in which 7 the said crystals are suspended. The water formed by the reaction is driven off during the I course of the reaction. This mixture of 208 pounds of the distinct crystals and 263 pounds of caustic forms a very heavy crystallinesuspension, whose physical nature facilitates easy separation in a centrifuge or other separatingv devices. Obviously, slurries of higher solid contents as well as, slurries oflower solid contents can be prepared, depending upon the desiredhandling characteristics .of the slurry.

It is believedthat the molten caustic is anhydrous by the time the granular crystals settle, as indicated. No fine bubbles. can be discerned in. the mother liquor. determination of water from a chemical analytical point of view by present-known methods is practically impossible, applicant cannot say positively that there is no water present.

The effect of higher temperatures is to increase the rate of the disappearance of the initial precipitate formed-on adding silica to molten caustic. This is also accompanied by a rise ginthe rate of iron pick-up from the vessel. For-this reason I prefer to stay below 900? E. Atthe other I end of the range, while the iron pick-up rate is low, the rate of recrystallizationof the precipitate formed is'extremely slow and, therefore,, I

But inasmuch as the .35 wprefer f to Ti operateiiaboveii 150 F. ff My rpreterred temperature" is' 800, which l fliidiielose to sbeing ran 'optimum' for the two-roppo'singeeflectsemenitioned.

"Variations of -my 'zciperati-ng eprocedurez'eare readily apparent to anyone 'rsleille'd dn the-rsart.

For example, a single? batch or sodium forthcsilicate 1 may be grown in a g-iven quantity of 'caustic soda, the mother #liquorcbf caustic sdda after the crystals have been removedrbeing dive'rted to'sorne other'purposeiand a rreshebatch of:caustic taken. Alternately; the-:motherliouor canbereturned to areactionwesselfthe depletion of caustic 'due-to' formation 'of-ia-sddiumeorthosilicate made up'bythe iintroduction ofifresh caustic 'soda' and" another crystallizatiompeirormed. Inanother modification, a series ofi-iron vessels may be "used, the firstone using fresh ealistic Iv soda as the starting material, the seo'ond (mereceiving the mother liquor from:the operationtin the first vessel plus -fresh-"caustic,= the thir' d one receiving the-mdther -=liquor from the operation in the second vessel, =andr-soc'on. this means crystals of varying degrees of purity and whiteness' could be obtained as each succeding'vessel would have a larger amount'uifiron and other impurities. -"In -"still another'rmo'dification the mother liquor from any crystallization can ..be run 'to'a suitable vessel forf'settling Whilerin fthe molten state to separate any precipitated-iron or other impurities .that may have been picked up, the clear liquor then being converted to the green state by properoheating or oxidation-land returned. to the crystallization vessel.

It is preferred to use silica .in a very finely divided state-and'cf"akhiglrpurity; such as a 200 mesh silica flour made. fromiigla'ss sand. iPar- :ticles -o,f. .silica iof this a size fdissolve .almostr'instantaneously in the molten caustic; grade silica does not add appreciablenimpuritiesi tot the melt. However; much coalser silloa can *beremployed up to anclaincludiug coarse Sdfidfi thflibfll}! ,efli'ect'being thatthe rate ofrsolution Willibeifionsiderably slower and'more-timennust betallotted for the completion of the reaction.

A continuous mode of operation can be employed if the choice is made to sacrifice uniformity of crystals. By such a mode silica can be added continuously or in very small increments to the molten caustic, preferably anhydrous caustic, in a vessel provided with an agitator, the slurry continuously withdrawn, the crystals centrifuged or otherwise separated from the mother liquor; the mother liquor thereupon returned to the said vessel together with fresh caustic in an amount suflicient to make up for the depletion. The crystals exhibit a wide range of sizes, from relatively coarse to extremely fine. However, for some purposes such a range of crystal sizes may be satisfactory and some classifications of the crystals accomplished by using a centrifuge f the solid bowl type. By such an expediency the finer crystals would remain in the mother liquor and be returned to the original vessel.

In order that the invention may be more clearly understood and readily carried into efifect, a method of operation in accordance therewith will now be described in somewhat greater detail.

The following example serves to illustrate the invention. It is given purely by way of an example in a somewhat greater detail than the description supra and without limiting the scope of my invention to the specific details set forth therein.

I .Emamfile I "M505ipounds'iofrmoltenrcausticfrnom a r-finishing spot: inithecmamnactureoofccaustic's soda. andoacccordinglyrinrthe tgreenstate, OIKOFHtIgIBBIl color cwereiaddeditmamironrpottfitted witlraanaagitator. i'llhercanstics soda'rwasi initiallyeirtcBZOfirF. F'Afftel afidditionztotthecoeldssmall s2p0t, ';the itemperature 210i thetmcltenvcaustic was broughtxtoiililfl Fr aby meansnot.iauburnernandnthermeltrsmairrtained-sat r2800frFithroughouttthe;entire-run. T ne-agitator swass startedzaaird fhtl rpoundsof? silica :a'iour added.

This silica reactedrwith thetzcausticisodayrwith athecevolut'ion of water .vaportand a' veryi finely rdivided precipitate could v *bez notice'd rin'vtheupot. 'Agitat'ioniwas continued zuntil' these small spara-ti'cles. had-rgrovvn sufiiciently rin size 'to obecome ggranular. "i Testz samples "of the agitatedi mixture 'i'werei removed from ttime" to 1 time from xthe IP01, "sbyx-meansropawsmall iadle, sandobservationrmade tthereorr of z-the time-required tor r the: particlesrto :.:settle$ substantially: completely. W'henotherparzrticlesu'had ibccomerlarge: enough 'to settlekinsap- .i-proximately. 10 secondsaanOther adiclitioncof 18.8 t poun-dsi -ofosilica waszthen rmaidemandcagitation continuedsuntiluther'settlingitest. agaim indicated 1 IllhlPBJtiOiI-BS settledslnt 10 seconds. This; was. conztinu'cdsuntil WA .-.-poundsoof si1ica= flour: hadnbeen :radded. intervalsrirbetweeniaadditionsvvaried eiromi lfl totailll' mirrutescunder the conditions-mt e'ag itation and? temperatureeabovexset-rout. l'l he total etime i'frorni 'theainitial 'z-addition "of silica? to -thetcompletionsof: the?reactionnwastil hours. At tithe end ..=0f Hthist. time '2 the li-pot :1 contained ea very 'crzheavyt'slurry of 1 -208 opounds cot iarge-t crystalseoi ssodiunriorthosilicate up 'toaaboutwlc mesh in size.

mhese rcrystals wverer'readily: separatcd inset-con rtriruge from thezmotheri'iliquonof moltencaiustic.

They were of a substantially white color andoonv\ tainecl does: then: IOQ parts FezGzrper "million.

Or;. .this"calculates to a' molan ratiofof. NazO-iSiOZ 1: sa is. zzichis comparesrwit-h; arratidtortliaifiim cot-i2. I'D-he. di'fierence: .intthe ratios; isaaccounted fort'byr the slight amountiof' caustic'zso'da adhering to the crystals. As seen from Figures 2 and 3,

the sodium orthosilicate product of this example is of a definite crystalline structure and in the hexagonal form; the particles are glass clear under the microscope and white to the naked eye. The comparison of the X-ray crystal pattern thereof, represented in Figure 1, with that of the dry process so-called sodium orthosilicate represented in this figure discloses that the two are not identical.

Other modes of applying the principle of my invention may be employed instead of the one explained, change being made as regards to the means and the steps herein disclosed, provided the steps are within the scope of the following claims.

I claim as my invention:

1. In the method of making crystalline anhydrous alkali metal orthosilicate in which the mol ratio of alkali metal oxide (MEzO) to $102 is 2 to l, the steps of reacting a quantity of a member of the group consisting of silica and the soluble silicates whose ratio of MEzOZ S102 is liquor.

said body of molten caustic alkali and maintaining the said heat range, expelling the additional water of reaction and precipitating outadditional fine particles, then repeating said'addition of more of such reactant with expulsion of 7 water at least once more, the quantity of said body being suilicientlygreat to maintain the presence of a caustic alkali mother liquor after the addition of all the siliceous reactant and after the completion of the reaction, then mechanically separating the granular crystals in the substantially anhydrous slurry from the caustic alkali mother liquor.

'2. In the method of making crystalline 'an hydrous sodium orthosilicate in which the'mol,

ratio of NazO to S102 is 2 to 1, the steps 'of intermittently introducing a member of the group consisting of silica'and the soluble sodium silicateswhose-NazO: SiOz ratio is less than in sodium orthosilicate in a pulverized condition with iron and maintained at'a temperature f 750 F. to 900 F. and ma condition termed the "-green state, the quantity of said body; being sufliciently great to maintain the presence of a caustic alkali mother liquor after the addition of all the siliceous-reactant and after the completion'of the reaction,-expellin'g the Waterof reaction resulting from each increment of siliceous material, precipitating fine particles'of sodium orthosilicate out of said body of mother liquor, agitating to cause the fine. particles anhydrous caustic soda, said caustic soda being making crystalline anin contact-with iron and maintained ata .tem-

:perature rangew'of 750 to 900 F. andint-a condition termed the-green state," the: silica being introduced at interspered intervals, in

aliquot 'portionsof substantially +25% of the v'total'amount added, the quantityof said body being sufliciently great to maintain the presence of a caustic, soda mother liquor after theaddition ofall the-silica and after the completion of-the reaction, expelling the water of reaction :.from each increment, precipitating out'fine particles of sodium orthosilicate from said body of into abody of molten caustic soda in contact mother liquor, agitating for aperiod of time be' tween substantially 40-80 minutes betweenadditions-to cause thefine; particles'formed after said additions to remain-in suspension and :be-

come granular before the next addition; 'thetotal amount of silica adding being sufficient .only-,t0

form a slurryof the sodium orthosilicate and then mechanically separating the said crystals from the molten caustic soda mother liquor.

Mellor; Inorganic and Theo'retical Chem -BRAZIERK. BEECHER. "JREFERE NCES CITED-"1 I The following references are of record in the ,file of this patent:

UNITED STATES PATENTS Number Name v v Date 2,142,265 Diekmann Jan. 3, 1935,

' [F N PAT S 1 Number "Country 7 fDa'te 484,547 Great Britain May 6, 1938 l O'II-IER REFERENCES istry, vol. 6, page 332. Pub. by Longmans, Green and C0., London'(l925). I 1

' Hou, Manufacture of Soda, A. C. S; Monograph 65, pages 110, 206. Pub. by Chemical Catalog'Co., New York (1933).

Hackhs Chemical Dictionary; third edition, page 389'. 'Pubr by the Blalnston Co., Philadelphia (1944). 7 w 

